Certain foodstuffs such as corn and peas have relatively high sulfur content compared to other foodstuffs. These foodstuffs, when packed in steel cans including tin-free and tin-plated steel cans have a tendency to stain the inferior surface of the can due to sulfur reaction with iron resulting in a dark iron sulfide stain. This staining, although harmless, gives an undesirable appearance when the can is opened.
It is known that staining can be reduced by incorporating zinc oxide pigment in the coating composition that is applied to the interior surface of the can. Typically the zinc oxide is milled with the resinous vehicle of the coating composition to form a dispersion. However, when the zinc oxide is milled with a carboxylic acid group-containing polymer such as (meth)acrylic polymer for aqueous-based compositions, the resulting dispersion has poor stability in that it gels on storage and can often gel when incorporated into the coating composition.